In recent years, regulation for exhaust gas has become stricter and lean burn engines, direct injection engines, and the like have become widespread. In this situation, a nitrogen oxides (hereinafter referred to as NOx) occlusion catalyst which can effectively purify NOx in exhaust gas in a lean atmosphere has been put into practical use. As NOx occlusion components used for the NOx occlusion catalyst, alkali metals such as K, Na, Li, and Cs, alkaline earth metals such as Ba and Ca, rare earth metals such as La and Y, and the like are known. In particular, Ba has been widely used from the beginning of practical use of the NOx occlusion catalyst. The addition of K excelling in the NOx occlusion capability in a high temperature range has been tried in recent investigations.
Such a NOx occlusion catalyst is usually formed from a catalytic material containing a NOx occlusion component such as an alkali metal and/or an alkaline earth metal (hereinafter referred to as “type of alkali metal”) loaded on a carrier of an oxide-type ceramic material such as cordierite or a metallic material such as an Fe—Cr—Al alloy.
However, these carriers tend to become degraded by being corroded with a type of alkali metal, particularly with Li, Na, K, Ca, or the like, which is activated by high temperature exhaust gas. If degradation proceeds, a honeycomb carrier may crack. In addition, the catalyst performance decreases over time due to reaction of the carrier material with a type of alkali metal.
In J-P-A-2-83254 is disclosed a honeycomb structure having high thermal shock resistance. The honeycomb structure is obtained by forming a honeycomb formed body from a sheet formed body prepared from a raw material containing mullite and at least cordierite added thereto, then firing the honeycomb formed body at a temperature of melting cordierite to bond mullite via molten cordierite.
The invention disclosed in this patent application, however, does not aim to use it as a carrier but aims to provide a honeycomb structure having high porosity and thermal shock resistance that can be molded at a low temperature inexpensively. Therefore, problems of the honeycomb structure carrying a type of alkali metal catalyst are not disclosed.